The present invention relates to an oxidation catalyst comprising a substrate and an oxidation coating of platinum (Pt), palladium (Pd), cobalt (Co), iron (Fe) and cerium (Ce) applied to the substrate. Furthermore, the invention relates to a method for producing such an oxidation catalyst and receiving exhaust gas from internal combustion engine.
For automotive applications, an oxidation catalyst is arranged normally within the exhaust gas system for oxidizing unburned hydrocarbons and carbon monoxide (CO) present in the exhaust gas. A conventional oxidation catalyst comprises a substrate containing aluminum oxide (AL2O3), e.g., an alumina washcoated cordierite substrate and a coating which comprises platinum (Pt). Such an oxidation catalyst is referred to an a Pt/alumina-catalyst or Pt catalyst.
If a particulate filter is already disposed in the exhaust pipe, the oxidation catalyst is normally placed upstream of the filter and is used to increase the exhaust gas temperature for the purpose of filter regeneration. Because a conventional Pt/alumina-catalyst has little significant catalytic activity at temperatures below 100° C., low-temperature catalysts were developed with an improved conversion rate at temperatures below 100° C. representing cold-start conditions of internal combustion engines. To improve the conversion rates below temperatures of 100° C., platinum-palladium coating on aluminum substrate were introduced. Such an oxidation catalyst can be named a Pt—Pd/alumina-catalyst or Pt—Pd catalyst.
A mixed Pt—Pd/alumina-catalyst for CO oxidation under cold-start conditions is described in the WO 96/39576. The disclosed catalyst is able to oxidize CO at near room temperature, but only if using zeolite as a water trap and a hydrocarbon trap to avoid catalyst poisoning by water and hydrocarbons. This catalyst is also vulnerable to sulfur compounds reacting with the palladium addition and is designed primarily for gasoline vehicles using low sulfur fuels. Similar Pt—Pd catalysts are also described in the U.S. Pat. No. 5,128,306 and JP 2005248787. Another advantage of such a bimetallic Pt—Pd/alumina-catalyst in comparison to conventional monometallic Pt/alumina catalyst is its higher thermal stability. Exhaust catalyst components can reach a temperature 650° C. during filter regeneration of a particulate filter. Furthermore, bimetallic Pt—Pd/alumina catalyst is also less expensive due to lower cost of palladium as compared to platinum. Yet another advantage of bimetallic catalysts is its tolerance to poisoning by CO adsorption at low temperatures as compared to monometallic Pt standard catalyst. Low temperature operation results in higher concentrations of CO.
The properties and advantages of such mixed Pt—Pd oxidation catalysts were, e. g., published in “Bimetallic Pt/Pd diesel oxidation catalysts” (Morlang, A.; Neuhausen, U.; Klementiev, K. V.; Schuetze, F.-W.; Miehe, G.; Fuess, H.; Lox, E. S.; Institute for Materials Science, Darmstadt University of Technology, Petersenstr. 23, Darmstadt, Germany. Applied Catalysis, B: Environmental (2005), 60(3-4), 191-199. Publisher: Elsevier B. V). One disadvantage of such oxidation catalysts is that typically the amount of NO2 is significantly higher after the exhaust gases have passed such a catalyst because that oxidation catalyst is able to oxidize the NO present within the exhaust gases to NO2. This effect is much more obvious with diesel engines than with gasoline engines because of the lean engine operation mode of diesel engines, i.e., the excess of oxygen. Unfortunately, Pt—Pd catalysts also increase NO2 emissions as compared with Pt catalysts. Increased NO2 emissions are an impediment widespread use of oxidation catalysts.
Technologies such as Selective Catalytic Reduction (SCR) using ammonia or urea and Lean NOx Traps (LNTs) are designed for the reduction of both NO and NO2 and not specifically targeting NO2. For example, JP 2005023921 discloses an exhaust emission control device to reduce NO2 by having an oxidation catalyst arranged at the front stage of an SCR catalyst. A bypass flow path is provided whereby exhaust gas is guided to the SCR catalyst while bypassing the oxidation catalyst. When excessive NO2 is produced by the oxidation catalyst, part of the exhaust gas is guided via the bypass flow path to the SCR catalyst thus bypassing the oxidation catalyst. This reduces the amount of the NO2 produced by the oxidation catalyst. JP 2005002968 shows a similar design.
JP 2005009407 shows a different approach. A fuel-supply valve, a catalyst, and a particulate filter are provided in this order in the path of an engine exhaust gas. The upstream end of the catalyst has a high Pt loading and oxidation activity. The downstream end of the catalyst has a high Pd loading in order to reductively eliminate NO2 formed by the platinum.
JP 2004353619 discloses a method to reduce NOx to N2. The system is equipped with a discharge device installed in an exhaust pipe in which exhaust gas is discharged and making ozone (O3) by a plasma discharge in oxygen (O2) and nitrogen monoxide (NO) contained in exhaust gas to react and generate nitrogen dioxide (NO2). A nitrogen dioxide (NO2) reduction catalyst is installed in a later part of the discharge device in the exhaust pipe and is reducing nitrogen dioxide (NO2) to nitrogen (N2). The disadvantage of active NO2 suppression is that these methods require reductant, active strategy and/or additional devices.